1. Field of the Invention
The present invention relates to an apparatus for detecting an original using a light-emitting element and a light-receiving element.
2. Related Background Art
Conventionally, as a document detection apparatus provided to an original size detection apparatus for a copying machine, one disclosed in U.S. patent application Ser. No. 930,911, and the like is known. More specifically, in order to detect an original placed on an original glass, this apparatus has a plurality of detection units each consisting of a light-emitting element, arranged above an original glass, for illuminating an original and a light-receiving element, arranged below the original glass for receiving light reflected by the original. In general, an original size is detected in accordance with outputs from the detection units at three positions on an original table.
FIG. 1 shows a conventional apparatus. A sensor 110 is a reflection type sensor including a light-emitting element such as an LED and a light-receiving element such as a photodiode. The sensor 110 emits light upward, and detects light reflected by an original 102 placed on an original glass 101, thereby detecting the presence/absence of an original.
In this case, in each detection unit, a gain (amplification factor) of the light-receiving element is adjusted to determine the presence of an original when it receives reflected light exceeding a predetermined level. An original size is detected in accordance with the presence/absence of an original at the respective points (three points).
However, in the conventional image forming apparatus, since the gain of the light-receiving element is fixed in advance, the presence/absence of an original may often be erroneously detected due to temperature characteristics of the light-emitting element and the light-receiving element or a variation in characteristics of individual elements.
Note that Japanese Patent Application No. 62-28442 discloses a technique in which in order to determine the presence/absence of an original, the original is exposed with an exposure lamp, light reflected by the original is read by a CCD, and a slice level for binarizing the output from the CCD is adjusted. However, this application does not disclose an arrangement in which a plurality of detection units each including a pair of a light-emitting element and a light-receiving element are arranged, and sensitivity of the light-receiving element of each detection unit is adjusted.
The sensor 110 in FIG. 1 receives light reflected by the original 102 to perform a detection operation of an original. For this reason, if a detection portion 102-A of the original 102 includes an image having a high density (e.g., black image), the sensor erroneously determines the absence of an original. If detection power of the sensor (light-receiving unit) is increased to detect a dark original as much as possible, a pressing plate 103 above the original glass may be detected or a detection error may be caused due to detection of external light if an original is absent.